The Late Archaean bonanza: metallogenic and environmental consequences of the interaction between mantle plumes, lithospheric tectonics and global cyclicity

Abstract

The Late Archaean records periods of intense magmatism and the development of prodigious metallogenic provinces of Ni, Fe, CuZn and Au deposits. In particular, the period 2.74-2.66 Ga represents one of the most widespread episodes of ultrabasic and basic volcanism preserved in the geological record, as well as anomalously widespread granitoid magmatism. Extensive assemblages of this age, which comprise komatiites and komatiitic basalts derived from mantle plumes, together with tholeiites and calc-alkalic volcanic rocks, are preserved on most Late Archaean cratons. Intense submarine volcanism in plume-like environments resulted in rich komatiite-hosted Ni mineralization in continental-margin basins, and CuZn sulphide mineralization in extensional volcanic arcs. Peak submarine magmatism was accompanied by marine transgression and thereby flooding of previously exposed continental crust. Elevated hydrothermal activity and widespread su☐ic conditions in submarine basins are reflected by sulphide-rich carbonaceous sedimentary rocks, that contain the organic remains of bacterial communities, and banded iron formations (BIF). A major episode of mesothermal gold mineralization accompanied accretionary tectonics, as metal-and carbon-rich submarine volcanic and sedimentary successions were subducted or incorporated into nascent continental crust by 2.59 Ga.Comparison with Neoproterozoic and Phanerozoic tectonic and metallogenic patterns indicates that the period 2.78-2.59 Ga represents the first half of an ∼ 360 m.y. global tectonic cycle. This period records the breakup of a supercontinent and the opening and closing of marginal basins along long-lived convergent margins of the external ocean to that supercontinent. Enhanced magmatic events between 2.74 and 2.66 Ga were most likely the result of intrabasinal mantle plumes and a subsequent global plume-breakout event. Together, the plume events were responsible for the extreme environmental conditions during the Late Archaean relative to both the preceding and succeeding periods of Earth history. Interactions between mantle plumes and long-lived convergent margins of a Pacific-type ocean were responsible for the prodigious metal inventory of Late Archaean granitoid-greenstone terranes. Extensive convergent-margin and plume magmatism during that period, coupled with episodic periods of low-angle subduction underplating by oceanic lithosphere, may also have been the cause of development of the buoyant, continental mantle lithosphere that is responsible for the preservation of these highly mineralized cratons. It is also likely that the bonanza metallogenic provinces in the Witwatersrand basin and Paleoproterozoic orogens of West Africa and Laurentia-Baltica reflect interactions of mantle plumes with long-lived convergent margins of the external ocean.